Though I still use my D700 and the IQ is still quite good, I really like packing my Panasonic and Olympus m43 and their very small and light lenses. The D700 and the 28-300mm is a nice combo, but in a few short hours, it's as heavy and convenient as a ball and chain. The m43 and 14-140 or 14-150 does the same in a smaller package, nearly as good or equal image quality as the D700 and liberating when you pack and shoot it all day.
In my recent travels, it has been nice to be ignored because my camera is small and quiet. When I shot with my D700, I stood out like a sore thumb.
The only real issue is not the image quality, but the number of photos per battery, about 15-20% of my D700 but with a few small batteries in my pocket, it's just a matter of changing them out. Since I now do some filming, that's another bonus with the m43 cameras.
Price-wise, a new m43 body will probably be more than the D700, but shopping around on eBay you'll find some bargains. However, the lenses will be cheaper than the FX equivalent.
If you like the size and the weight of the D700/28-300mm go for it and you'll be happy. If you want something quiet, small, light-weight, go for the m43.